A femto base station refers to a subminiature mobile communication base station that accesses to a mobile communication core network through a broadband network installed within a building, such as a home or an office. In case of within a building, the coverage of a macro base station may be limited by an outer wall of the building, or the like, but in case where a femto base station is installed indoors in such an environment, it is possible to expand the coverage and enhance voice quality. Also, it has an advantage of effectively providing a convergence service between high-bandwidth wired and wireless lines.
The femto base station can be divided into a CSG (closed subscriber group) femto base station, and an open access femto base station based upon whether or not user equipment is accessible to the femto base station. In case of a CSG femto base station, the femto base station is accessible only by user equipment for which a service is allowed to the relevant femto base station, but in case of an open access femto base station, the femto base station is unrestrictedly accessible by any user equipment for which the femto base station provides a service. In particular, a femto base station white list is defined in 3GPP, and therein are stored CSG IDs for checking that the relevant femto base station is a CSG femto base station, and a CSG ID list for the relevant terminal to access to a USIM (universal subscriber identity module) card of a femto base station supporting terminal.
As a result, when the femto base station accessible during terminal movement is an open access or CSG femto base station but the terminal is allowed to access, handover is carried out to receive a high-bandwidth mobile communication service.
FIG. 1 is a structured diagram illustrating a mobile communication environment in which a femto base station is installed within the coverage of a macro base station as a prior art.
In FIG. 1, the base station included in a mobile communication system can be divided into femto base stations 120, 130 and a macro base station 100 based upon the cell coverage or deployment method. The coverage of femto base stations 120, 130 has a smaller size than that of the coverage of macro base station 100. All or part of the coverage of femto base stations 120, 130 may overlap with the coverage of macro base station 100. User equipment (UE) 110 within the macro base station 100 generally performs a communication with the macro base station 100, and the macro base station 100 is connected to a mobile communication core network 160 by a dedicated line However, when the femto base station 120 is deployed in a closed position to the user equipment 110, that is, when the user equipment 110 is deployed in the coverage of the femto base station 120, the user equipment 110 performs a communication with the femto base station 120. The femto base station is connected to a femto base station controller 150 by a wired line through a broadband network 140, and the femto base station controller 150 is connected to the mobile communication core network 160.
When another femto base station 130 is searched out while the user equipment 110 is moving in the femto base station 120 being communicated to each other, it is first determined whether or not the terminal is accessible to the searched femto base station 130, and CSG IDs being periodically transmitted from the femto base station are checked in case of a CSG femto base station, and then, if the relevant femto base station is registered in the white list of the user equipment 110, the a signal quality of the relevant femto base station 130 is checked, and handed over if the signal quality is good.
However, the number of terminals simultaneously accessible to a femto base station is typically a few, about 3 to 5, and therefore, the ratio occupied by a terminal in the overall traffic amount of a femto base station is very high. Accordingly, a frequency resource amount required by the femto base station may be greatly varied during the handover of a terminal.
Accordingly, when a conventional terminal handover method is used by considering only a signal quality of the femto base station to be moved, it may happen to exceed the number of accessible terminals or have insufficient radio resources required for communicating with terminals by the relevant femto-cell if many terminals have been already accessed to the relevant femto base station to be moved by a terminal; on the contrary, it may happen to waste radio resources if not so many terminals are accessed to the femto base station currently accessed by a terminal.
To solve such a problem, there is a method for rearbitrating a frequency resource assignment amount between femto base stations assigned by a femto base station controller or core network, based upon a number of terminals connected to the femto base station and an amount of traffic, to reduce radio resources, but the method has a disadvantage of consuming a relatively long time because the deployed femto base station controller or core network become a subject to search all femto base stations even a femto base station for which resource assignment is not required, and implement a radio resource reallocation by considering all the factors, such as the number of terminals connected to each femto base station, the amount of traffic, and the interference between femto-cells. Moreover, the method has a disadvantage that it is difficult to cope with the situation frequently occurred locally for a short period of time, such as the handover of a terminal between femto-cells, since the radio resources assignment for a femto-cell by the femto base station controller is carried out with a relatively long period and the radio resources are reassigned during a specific event such as femto-cell installation or the like.
As a result, there is a need to rearbitrate frequency resources that have been already assigned between the relevant femto base stations only for the femto base stations required for reallocating radio resources, that is, the serving femto-cell to which a terminal has been previously accessed and the target femto-cell to which a terminal will be handed over, thereby reducing the time consumed for reallocating radio resources as well as effectively using the radio resources.